Widely known in the prior art are the processes for the production of polymer materials involving coagulation of polymer dispersions by ionic deposition, electrodeposition, drying and thermosensibilization. The indispensable prerequisites for implementation of these processes include constant properties of polymer dispersions, high concentration of polymer dispersed phase from 40 to 60 wt.-%, strict control of temperature and time parameters, employment of coagulants.
These processes are good mainly for the production of polymer films. Control of structure and, consequently, physicomechanical properties of such materials in the desired direction is hardly attainable due to the technological difficulties in maintaining said parameters within the necessary interval.
Known in the prior art is a process for the production of a microporous polymer leather-like watertight sheet material (U.S. Pat. No. 3,376,158) which involves applying an aqueous polymer dispersion-latex to a porous flexible fibrous substrate, coagulating polymer latex by freezing at minus 100.degree. to minus 10.degree. C., and drying the resulting product.
The process envisages treatment of polymer latexes capable of coalescing within a temperature interval from 10.degree. to 100.degree. C. and characterized by a secant module which is not under 22 Pa at a 5% tension. Concentration of the polymer dispersed phase ranges from 10 to 75 wt.-%, when a layer of latex 0.07 to 2.3 mm thick is deposited on the fibrous substrate. Before depositing, the latex is thickened to 10-40 P with caseinate and ammonium alginate, methyl cellulose and sodium polyacrylate. The polymer latex is also treated with crosslinking agents, accelerators and crosslinking activators, thickeners and fibrous additives. All these ingredients, particularly in presence of an additional destabilizing factor, in this case freezing, tend to create an orderly structure and speed up this process, thus forming a microporous polymer structure. It should be noted, that this structure is also largely defined by the structure of the porous fibrous substrate to which latex is applied.
In realizing the above process, freezing and resulting coagulation form a discrete polymer structure on the substrate. Owing to the effect of crosslinking agents and the coalescing capacity of the used polymers said structure is transformed by drying into a thin polymer coating of irregular thickness. This structure is then improved by repeated deposition of latex on the resulting polymer coating, followed by freezing and drying.
The above-described process denies the possibility of directional control over the structure of the microporous polymer material since freezing combined with the use of above-mentioned additions exerts but a destabilizing effect, so that said structure depends predominantly upon the structure of the fibrous substrate and the drying conditions. The irregularity of thickness of the material structure is apt to impair the physicomechanical properties.
There is another known process for the production of a porous polymer material in the form of a silicone elastomer sponge with mostly closed pores from an aqueous polymer dispersion-emulsion of polyorganosyloxane with pH=9-11.5 and a molecular weight over 10.000 in the presence of calloidal silica and an organic compound of tin (EP, 0097914). This process involves freezing of the silicone emulsion to form a conjugate-dispersed polymer system in the form of an article, thawing said article to produce a wet elastic sponge article, and drying the wet article until water is removed therefrom.
The silicone elastomer sponge is produced from a concentrated emulsion of polyorganosyloxane (30-60 wt.-%) which is frozen at minus 18.degree. C. for 24 h followed by thawing the frozen article at room temperature for 6 h and drying it at 70.degree. C. It should be noted that a mandatory prerequisite for sponge production is the use of emulsion containing a filler (colloidal silica) and an organic compound of tin.
The filler forms the structure of emulsion due to sorption of polyorganosyloxane which reacts with the organic tin compound, ensuring the formation of sponge polymer structure at freezing-thawing stages. To enhance uniform distribution in the emulsion, the filler is introduced in the form of a stabilized aqueous dispersion of silica which later will ensure a more regular structure of silicone elastomer sponge. It is precisely the filler and the organic tin compound which allow the sponge structure of the article to be fixed before drying. The closed-pore silicone elastomer sponge is produced only when drying a wet sponge article whose wet polymer walls of pores stick partly together. The sticking effect shows itself to a maximum when producing film-like specimens whose porosity can be improved by forming the emulsion before freezing.
The process referred to above is simple from the viewpoint of technology and releases no toxic materials. However, said process fails to change the structure of silicone sponge within broad limits to suit various applications. This is attributable to the fact that freezing is a destabilizing factor which favours coagulation of polymer dispersion so that the structure of sponge depends on stability of emulsion, molecular weight of polymer, the quality and quantity of the filler and organic tin compound. The effect of these parameters denies the possibility of producing silicone sponge with a regular structure throughout its volume, and with valuable physicomechanical properties.